The present invention relates to a solar ray collecting device in which the sunlight, focused by the use of a Fresnel lens, is effectively guided into an optic cable.
The present applicant has previously proposed a solar ray collecting device comprising a large number of lenses. The solar rays focused by the lenses are guided into the optical conductor cables. The solar rays guided in such a way are transmitted through the optical conductor cable onto an optical desired place.
In the above mentioned solar ray collecting device, when the numerical aperture angle of the lens is large, the sun's image, focused by the lens, is small. Consequently, a optic cable of small diameter can be used. This is the advantage of its size. However, the rising angle at the circumferential portion of lens is large, and therefore the amount of light reflecting at this portion is large and the focusing efficiency is not good. The incident angle of the optic cable is also large. Consequently, the amount of reflection is large at the light-receiving end of the optic cable and the solar ray guiding efficiency into the optic cable is not good. Furthermore, the solar rays entering the lens are reflected onto the light-emitting end of it and returned into the lens. Thereafter, the reflected solar rays propagate inside the lens. For this reason the incident rays cannot be guided effectively into the optic cable.
On the other hand, when the numerical aperture of the lens is small, the rising angle at the circumferential portion of it is small. Consequently, the amount of reflection is small at this side and the incident angle for the optic cable is small so that the amount of reflection at the light-receiving end of the optic cable is also small. Therefore the collection of solar rays becomes more efficient. On the contrary, the sun's image focused by the lens is large. Consequently, the diameter of the optic cable needs to be large. Consequently the cost of the optic cable becomes high. Those matters are the weak points of the prior art.